FIELD OF THE INVENTION
This invention relates to a so-called carrier relay system in which the quantities of electricity detected at individual terminal stations of a power transmission system are transmitted between the terminal stations for the purpose of protection of power transmission lines, and more particularly to a carrier relay system in which optical fibers incorporated in an overhead ground wire are used as a signal transmission medium.
Two types of carrier relay systems are widely known and employed in this field of the art. In the former type of power line transmission type, power transmission lines themselves are used as signal transmission routes, while, in the latter type or microwave transmission type, microwave circuits are used for the signal transmission. However, the former type has such a drawback that apparatus including power line connectors of large capacity and large scale are required. The latter type has also such a problem that approval by the administrative agencies of the state is required for the use of the microwave circuits as signal transmission routes, and, thus, the microwave circuits cannot be adopted freely for this purpose.
On the other hand, the modern progress of technology promotes extended use of optical fiber cables, and an overhead grounding wire combines with optical fiber (referred to hereinafter as an OPGW) having an optical fiber built in an overhead ground wire extending in parallel with the power transmission lines has already been developed and put into practical use.
The economomic advantage of employment of such an OPGW is comparable to that of the microwave circuit, and, therefore, there is now a strong tendency toward positive utilization of the internal optical fiber of the OPGW as a signal transmission route for protective relays including PCM carrier relays.
In a conventional microwave circuit, the adverse effect of fading due to a natural phenomenon such as a lightning discharge is inevitable. It has therefore been a common practice that a signal transmission route associated with a power transmission line to be protected is divided into two signal transmission routes laid in respectively different areas. (Such an arrangement will be referred to hereinafter as a duplex route arrangement.) That is, this duplex route arrangement is based on the expectation that, even when one of the two transmission routes may be affected by a lightning discharge, the other will not be affected and will remain sound.
However, the internal optical fiber of the OPGW is not adversely affected by the fading nor the electromagnetic action of lightning. Also, since the OPGW itself is supported on the same steel tower on which the associated power transmission line to be protected is also supported, there is no necessity to lay another OPGW as a signal transmission route extending in an area different from the area in which the power transmission line to be protected extends. Therefore, it is the present tendency that two optical fibers are incorporated in the OPGW when it is desired to duplex the signal transmission route for improving the reliability of the relay system.
A relay system has been proposed in which optical-fiber signal transmission routes arranged in duplex on the basis of the above idea are provided so that, even when a fault occurs in one of the optical fibers, the faulty signal transmission route can be switched over to the other optical fiber. Such a relay system is disclosed in a paper entitled "Current Differential Type Protective System for Optical PCM Transmission" reported in the General Meeting of the Institute of Electrical Engineers of Japan, 1983, No. 1218.
In the protective relay disclosed in the paper above described, occurrence of a fault in only one of the optical fibers arranged in duplex is detected by, for example, an error detecting circuit which acts to change over an optical fiber change-over or selector switch, so that the relay continues to operate on the basis of data received via the stand-by optical fiber. However, in the event that the steel tower supporting the power transmission line and the associated OPGW falls down for some reason, both of the optical-fiber signal transmission routes arranged in duplex will be simultaneously damaged. Therefore, the proposed protective system is defective in that concurrent occurrence of grounding and short-circuit troubles results in impossibility of the desired relay operation or malfunction of the protective relay, and the protective relay cannot achieve its primary function of system protection.